In gas turbine engines, such as aircraft engines, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel in a combustor. The mixture is then burned and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas expands through the turbine which in turn spins the shaft and provides power to the compressor. The hot exhaust gases are further expanded through nozzles at the back of the engine, generating powerful thrust, which drives the aircraft forward.
Because engines operate in a variety of conditions, foreign objects may undesirably enter the engine. More specifically, foreign objects, such as large birds, hailstones, sand and rain may be entrained in the inlet of the engine. As a result, these foreign objects may impact a fan blade and cause a portion of the impacted blade to be torn loose from the rotor, which is commonly known as fan blade out. The loose fan blade may then impact the interior of the fan casing causing a portion of the casing to bulge or deflect. This deformation of the casing may result in increased stresses along the entire circumference of the engine casing.
In recent years composite materials have become increasingly popular for use in a variety of aerospace applications because of their durability and relative lightweight. Although composite materials can provide superior strength and weight properties, and can lessen the extent of damage to the fan casing during impacts such as blade outs, designing flanges on structures fabricated from composite materials still remains a challenge.
Laminated composite structures generally have superior strength in-plane due to the presence of continuous reinforcing fibers. However, issues may arise when attaching a secondary structure to an interposing flange located about the body of the composite structure, as opposed to about an end of the composite structure. Such issues are due to a general lack of continuous fibers at the points of attachment, or joints, between the flange and primary composite structure. This, in addition to significant out-of-plane loads caused by the weight of the secondary structure, may result in a weak attachment joint that is susceptible to damage from increased stresses, such as those resulting from a fan blade out or those inherently present due to the weight of the secondary structure.
To address such weaknesses at the point of attachment, it may be desirable to provide supplementary reinforcement to the joints of the mounting flange, such as additional fibers or metal brackets. However, with the addition of these reinforcements, the weight-saving benefits provided by using composite structures can be significantly reduced. Moreover, even with additional reinforcements, the mounting flange may still not be strong enough to adequately support the weight of the attached secondary structure, with or without the additional stresses caused by a blade out. Ultimately, continuous stresses on the already weakened flange may result in catastrophic failure to one or more of the primary composite structure, the attached secondary structure, the engine or the aircraft.
Accordingly, there remains a need for methods for reducing stress on composite structures having mounting flanges that provide the desired attachment without the previously described failure issues.